This invention relates to a current driving system for the stator windings of a direct current brushless motor in which magneto-electrical conversion elements, an addition network and a switching circuit are employed instead of the conventional brush and commutator, and wherein sequential currents having predetermined, non-overlapping flow angles are applied to the stator windings in response to an angular rotor position detections.
In a direct drive phonographic turntable system wherein the drive motor necessarily at very low r.p.m.'s, i.e. 33 or 45, exceptionally smooth and flat speed and torque curves are essential. Brushless d.c. motors having permanent magnet rotors are well adapted to this purpose, and typically the number of rotor and stator poles are unequal to product smoother speed and torque curves. This creates problems, however, in designing circuitry to drive the stator windings too produce such smooth operation.
Where Hall effect devices are employed as angular rotor position detection elements they are spaced at predetermined intervals around the periphery of the stator. Such Hall elements are often not uniform in sensitivity, however, and thus produce unbalanced voltages and non-analogous outputs. In order to supply smooth and balanced currents having predetermined phase differences to the stator windings, the Hall element outputs are typically shaped into rectangular waveforms by differential amplifiers. The outputs of the differential amplifiers have flow angles of 180 electrical degrees with respect to the rotor, however, while flow-angles having shorter durations are needed to drive the stator windings owing to the unequal number of rotor and stator poles. If the stator windings are driven at flow angles of 180.degree. with respect to the rotor, the drive currents flowing in such stator windings are overlapped. This reduces the efficiency of the motor and increases the torque ripple, as contrasted to the smoother operation which results when the stator windings are driven by non-overlapping currents having shorter flow angles (120.degree. or 60.degree.).